Exhaust gas recirculation control valve

ABSTRACT

A vacuum operated valve responsive to exhaust back pressure controls recirculation of exhaust gases from the intake manifold exhaust crossover passage to the intake manifold induction passages.

United States Patent 1191 Kingsbury 1451 Sept. 10,1974

[ EXHAUST GAS RECIRCULATION [56] References Cited CONTROL VALVE UNITED STATES PATENTS [75] Inventor: William L. Kingsbury, Madison 1,051,690 l/ 1913 Colwell 123/1 19 A Heights, Mich. 2,851,021 9/1958 Couone... 123/119 A 0 3,507,260 4/1970 Walker 123/119 A [73] Asslgneez General Motors Corporation, ,939 2 1972 Hill 123 119 A Detroit, Mich. 3,662,722 5/1972 Sarto 123/119 A [22] Wed: July 1973 Primary Examiner-Wendell E. Burns [21] Appl. No.: 375,362 Attorney, Agent, or Firm-C. K. Veenstra Related US. Application Data 57] ABSTRACT [63] Continuation of April 1972' A vacuum operated valve responsive to exhaust back pressure controls recirculation of exhaust gases from (g1 the intake manifold exhaust crossover p g to the [58] Field of 123/119 A intake manifold induction passages 5 Claims, 5 Drawing Figures EXHAUST GAS RECIRCULATION CONTROL VALVE This is a continuation of application Ser. No. 244,378, filed Apr. I7, 1972.

SUMMARY OF THE INVENTION This invention relates to exhaust gas recirculation in an internal combustion engine and more particularly to a novel valve assembly for controlling exhaust gas recirculation.

Recirculation of exhaust gases has been developed as a method for reducing formation of oxides of nitrogen during the combustion process in an internal combustion engine. In general, it is desired to recirculate the exhaust gases at a rate proportional to the rate at which combustion air flows into the engine, and valves responsive to induction passage vacuum or throttle position have been utilized for this purpose.

It also has been recognized that if exhaust gases were recirculated through an orifice into a region of substantially atmospheric pressure in the engine air induction system, variations in exhaust back pressure would cause the exhaust gas recirculation rate to be proportional to the combustion air flow rate. However, such a system would require that the exhaust gases pass through at least a portion of the carburetor.

This invention provides a novel valve assembly utilizing the exhaust back pressure to recirculate exhaust gases at a rate proportional to air flow and in a manner which avoids recirculation of exhaust gases through the carburetor. In employing this invention, an exhaust gas recirculation passage is provided which extends from the engine exhaust passage to the engine air-induction passage at a point downstream of the engine throttle. An orifice is provided in the recirculation passage, and a valve disposed downstream of the orifice is operated to create a zone of substantially constant pressure in the passage irrespective of the wide variations in exhaust back pressure and induction passage vacuum. The rate of recirculation of exhaust gases through the zone thus is proportional to the rate of induction air flow.

In other valve assemblies recently proposed for controlling exhaust gas recirculation in accordance with exhaust back pressure, the diaphragm or other member responsive to back pressure has directly operated the valve member which controls the flow of exhaust gases. In the design of such a valve assembly, certain limitations are encountered because the back pressure generally is very low. In the valve assembly of this invention, on the other hand, the exhaust gas flow controlling valve member is not directly operated by the member responsive to exhaust back pressure; instead, the valve member is positioned by a member operated by an induction passage vacuum signal and the member responsive to exhaust back pressure controls that signal.

Several embodiments of this valve assembly are depicted herein, the details of which as well as other objects and advantages of this invention are set forth in the remainder of the specification and are shown in the drawings.

SUMMARY OF THE DRAWINGS FIG. 1 is a top plan view of an internal combustion engine inlet manifold having induction and exhaust gas crossover passages, an insert plate having an exhaust gas recirculation passage mounted on the manifold, and the exhaust gas recirculation control valve assembly mounted thereon;

FIG. 2 is a sectional view along line 2-2 of FIG. 1 showing the induction, exhaust gas crossover, and exhaust gas recirculation passages and also showing the throttle body portion of a carburetor mounted on the insert plate;

FIG. 3 is an enlarged sectional view, in elevation, of one embodiment of the control valve assembly. taken generally along line 33 of FIG. 1;

FIG. 4 is a sectional view, in elevation, of another embodiment of the control valve assembly; and

FIG. 5 is a sectional view, in elevation, of yet another embodiment of the control valve assembly which also shows, schematically, an additional control mode which may be utilized with this embodiment.

This invention is an improvement over the invention of Robert A. Bolton claimed in commonly assigned application Ser. No. 245,587 filed Apr. 19, 1972. The improved embodiment of this invention shown in FIG. 3 was invented by Paul B. Kuehl and is specifically claimed in commonly assigned application Ser. No. 242,420 filed Apr. 10, I972. The embodiment of this invention shown in FIG. 5 includes valve surfaces regulating both the air bleed orifice and the vacuum orifice; the additional control of the vacuum orifice was invented by Robert A. Bolton and is specifically claimed in the aforementioned application Ser. No. 245,587. The FIGS. 4 and 5 embodiments make use of the invention of Edward G. Day and Ernst L. Ranft claimed in commonly assigned application Ser. No. 220,036 filed Jan. 24, 1972. The FIG. 4 embodiment also makes use of an invention of Ernst L. Ranft and William F. Thornburgh claimed in commonly assigned application Ser. No. 314,569 filed Dec. 13, 1972.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 and 2, the combustion air induction passages for the engine are formed in part by an intake manifold 10 which has a pair of vertical primary riser bores 12 and 14 and a pair of vertical secondary riser bores 16 and 18. Riser bores 12 and 16 open to an upper horizontal plenum 20 connected forwardly (leftwardly as viewed in FIG. 1) to a pair of transverse runners 22 and 24 and connected rearwardly (rightwardly as viewed in FIG. 1) to another pair of transverse runners 26 and 28. Similarly, riser bores 14 and 18 open to a lower horizontal plenum 30 connected forwardly to a pair of transverse runners 32 and 34 and rearwardly to another pair of transverse runners 36 and 38.

Intake manifold 10 also has an exhaust crossover passage 40 which extends transversely from the left-hand side of manifold 10 beneath plenums 20 and 30 and receives a portion of the exhaust gases discharged from the engine combustion chambers.

An insert plate42 is secured on manifold 10 and has primary riser bores 44 and 46 and secondary riser bores 48 and 50 which meet, respectively, riser bores 12, 14, 16, 18 of manifold 10.

A carburetor 52 is secured on insert plate 42 and has primary throttle bores 54 and 56 which meet, respectively, primary riser bores 44 and 46 of insert plate 42. Carburetor 52 also has secondary throttle bores (not shown) which meet secondary riser bores 48 and 50 of insert plate 42. Throttles 57 are disposed in the carburetor bores to control induction air flow therethrough.

A bore 58 in manifold leads upwardly from exhaust crossover passage 40 to the first portion 60 of an exhaust recirculation passage formed in insert plate 42. The first portion 60 of the exhaust recirculation passage leads through a control valve assembly 62 to a second portion 64 of the exhaust recirculation passage. This second portion 64 divides into a pair of branches 66 and 68 which lead to the primary riser bores 44 and 46 in insert plate 42.

It should be appreciated that both portions 60 and 64 of the exhaust recirculation passage may be integrated in manifold 10 rather than in separate insert plate 42.

FIG. 3 shows one embodiment 62a of control valve assembly 62. Embodiment 62a comprises a valve body 70 having an inlet 72 for receiving exhaust gases from first portion 60 of the exhaust recirculation passage and an outlet 74 for discharging exhaust gases to second portion 64 of the exhaust recirculation passage. An orifree member 76 is disposed-across inlet 72, and a downwardly facing valve seat 78 is disposed between inlet 72 and outlet 74. A valve member 80, associated with valve seat 78, is formed on a hollow stem 82 carried by a pressure responsive diaphragm 84 and upwardly biased by a spring 86.

A passage 88 senses the pressure in the zone 90 between orifice member 76 and valve seat 78 and transmits the zone pressure to the chamber 92 above a diaphragm 94. Diaphragm 94 operates against the bias of a spring 96 is response to variations in pressure in zone 90 to position a pilot valve 98 disposed in hollow stem 82.

Hollow stem 82 has lateral openings 100 disposed in the chamber 102 between valve seat 78 and outlet 74 and transmits the induction passage vacuum present therein past valve 98 through upper lateral openings 104 to the chamber 106 below diaphragm 84. In operation, as the pressure in zone 90 increases, diaphragm 94 is depressed to open valve 98 and admit manifold vacuum through openings 104 to chamber 106, thereby decreasing the pressure signal in chamber 106. Diaphragm 84 is then pulled downwardly against the bias of spring 86 to displace valve member 80 from valve seat 78. This operation permits increased recirculation of exhaust gases. Upon a reduction in pressure in zone 90, spring 96 raises diaphragm 94 thus closing valve 98 and permitting the pressure signal in chamber 106 to increase as air is bled through a slot 108 in valve member 98 from the chamber 110 between diaphragms 94 and 84. Diaphragm 84 is then pushed upwardly by spring 86 to displace valve member 80 toward valve seat 78, thereby reducing recirculation of exhaust gases. in practice, diaphragms 94 and 84 will move in unison and, a substantially constant pressure will be maintained in zone 90.

The back pressure created in the exhaust passages such as 40 of an internal combustion engine is generally proportional to the square of the rate of combustion air flow through the engine induction passages. The rate of flow of exhaust gases from first portion 60 of the exhaust recirculation passage through an orifice such as 76 into a zone such as 90 of substantially constant pressure is generally proportional to the square root of the exhaust back pressure. Thus the rate at which exhaust gases are recirculated is generally proportional to the rate at which combustion air flows to the engine.

FIG. 4 shows another embodiment 62b of control valve assembly 62. It comprises a valve body 112 having an inlet 114 receiving exhaust gases from first portion 60 of the exhaust recirculation passage and an outlet 1 16 discharging exhaust gases to second portion 64 of the exhaust recirculation passage. An orifice member 118 is disposed across inlet 114, and an upwardly facing valve seat 120 is formed about outlet 116. A valve member 122, associated with valve seat 120, is secured on a stem 124 carried by a pressure responsive diaphragm 126 and downwardly biased by a spring 128.

The pressure in the zone 130, defined between orifice member 118 and valve seat 120, is applied through lateral openings 132 and a longitudinal passage 134 in stem 124 to a chamber 136 defined between the central portion 138 of diaphragm 126 and a dished diaphragm backing member 140. A chamber 142, defined between central portion 138 of diaphragm 126 and another dished diaphragm backing member 144, is maintained at atmospheric pressure by openings 146 extending through diaphragm 126 and backing member 140. The chamber 148 above diaphragm 126 is subjected to the pressure in the induction passage downstream of throttles 57 by a vacuum hose 150 which may be connected to carburetor 52, manifold 10, or second portion 64 of the exhaust recirculation passage.

In operation, as the pressure in zone 130 drops, central .portion 138 of diaphragm 126 is pushed downwardly by a spring 152. A valve member 154 secured to diaphragm portion 138 then is displaced from an air bleed opening 156 to admit air from chamber 142 to chamber 148. This increases the pressure signal in chamber 148, and spring 128 displaces diaphragm 126, stem 124 and valve member 122 toward valve seat 120 to reduce recirculation of exhaust gases. Upon an increase in pressure in zone 130, diaphragm portion 138 moves upwardly against the bias of spring 152 and valve member 154 reduces air flow through opening 156 into chamber 148. The resulting reduction in the pressure signal in chamber 148 displaces diaphragm 126, stem 124, and valve member 122 upwardly from valve seat 120, thereby increasing recirculation of exhaust gases. Embodiment 62b also maintains a constant pressure zone, 130, downstream from an orifice, 118, which receives exhaust gases at exhaust back pressure and thus provides recirculation of exhaust gases at a rate proportional to induction air flow.

F IG. 5 shows yet another embodiment 620 of control valve assembly 62. It comprises a valve body 158 having an inlet 160 receiving exhaust gases from first portion 60 of the exhaust recirculation passage and an outlet 162 discharging exhaust gases to second portion 64 of the exhaust recirculation passage. An orifice member 164 is disposed across inlet 160 and an upwardly facing valve seat 166 is disposed between inlet 160 and outlet 162. A valve member 168 associated with valve seat 166 is connected by a stem 170 to a pressure repilot valve 184 is connected by a stem 186 to diaphragm 180 and controls an air bleed through an orifice 188 into a vacuum chamber 190. A fitting 192 on chamber 190 is connected through hoses 194 and 196 to a source of vacuum such as that in carburetor 52 below throttles 57 of that in outlet 162. A second fitting 198 on chamber 190 is connected by a hose 200 to the chamber 202 above diaphragm 172.

In operation, upon-an increase in pressure in zone 174 diaphragm 180 is raised against the bias of spring 182 and valve member 184 reduces air flow through orifice 188 into chamber 190. As the pressure signal in chamber 190 decreases, diaphragm 172 is raised against the'bias of spring 204 and valve member 168 is displaced from valve seat 166 to permit increased recirculation of exhaust gases. Upon a decrease in pressure in zone 174, spring 182 depresses diaphragm 180 and valve member 184 permits increased air flow through orifice 188 into chamber 190. The increased pressure signal in chamber 190 is transmitted to chamber 202 and spring 204 lowers diaphragm 172 to displace valve member 168 toward valve seat 166, thus reducing recirculation of exhaust gases. Thus embodiment 620 also is effective to maintain a constant'pressure zone, 174, downstream from an orifice, 164, which receives exhaust gases at exhaust back pressure and thus provides recirculation of exhaust gases at a rate proportional to induction air flow.

If desired, a valve 206, responsive to temperature or some other engine or vehicle operating condition, may be disposed between hoses 194 and 196 to control application of vacuum to chamber 202 and thus to superimpose a supplemental control on recirculation of exhaust gases.

In some applications, it may be desirable to provide an additional surface 208 on valve member 184 to control the effective area of fitting 192 and thus control application of vacuum to chamber 190. In other applications, it may be desirable to connect hose 200 to fitting 192 and hose 194 to fitting 198 whereby valve surface 208 may control application of vacuum to chamber 202.

It will be appreciated that each embodiment of valve assembly 62 may be tailored to prevent recirculation of exhaust gases whenever the induction passage vacuum is very low, thus preventing any reduction in power due to charge dilution during wide open throttle operation.

I claim:

1. An exhaust gas recirculation control valve assembly for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, and an exhaust gas recirculation passage having a first portion extending from said exhaust passage and a second portion extending to said induction passage downstream of said throttle, said control valve assembly comprising a valve body having an inlet for receiving exhaust gases from said first portion of said recirculation passage, an outlet for discharging exhaust gases to said second portion of said recirculation passage, valve seat means formed between said inlet and said outlet, and an orifice formed in said inlet, valve means associated with said valve seat means for controlling flow of exhaust gases therethrough, and control means for positioning said valve means to maintain a substantially constant pressure in the zone between said orifice and said valve seat means, said control means including spring means biasing said valve means toward engagement with said valve seat means, a pressure responsive member connected to said valve means, means defining a vacuum orifice through which said member is subjected to the pressure in said induction passage downstream of said throttle, means defining an air bleed orifice through which said member is subjected to atmospheric air, a bleed valve associated with one of said orifices for controlling flow therethrough and thus regulating the control pressure created by induction passage vacuum received through said vacuum orifice and atmospheric air received through said air bleed orifice, pressure responsive means connected to said bleed valve and subjected to the pressure in said zone whereby upon an increase in pressure in said zone said pressure responsive means displaces said bleed valve to decrease said control pressure to thereby cause said pressure responsive member to overcome the bias of said spring means and displace said valve means from said valve seat means and increase recirculation of exhaust gases, and second spring means biasing said bleed valve to increase said control pressure whereby upon a decrease in pressure in said zone said second spring means displaces said bleed valve to increase said control pressure to thereby permit said first spring means to displace said valve means toward said valve seat means and decrease recirculation of exhaust gases.

2. An exhaust gas recirculation control valve assembly for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, and an exhaust gas recirculation passage having a first portion extending from said exhaust passage and a second portion extending to said induction passage downstream of said throttle, said control valve assembly comprising a valve body having an inlet for receiving exhaust gases from said first portion of said recirculation passage, an outlet for discharging exhaust gases to said second portion of said recirculation passage, valve seat means formed between said inlet and said outlet, and an orifice formed in said inlet, valve means associated with said valve seat means for controlling flow of exhaust gases therethrough, and control means for positioning said valve means to maintain a substantially constant pressure in the zone between said orifice and said valve seat means, said control means including spring means biasing said valve means toward engagement with said valve seat means, a pressure responsive member connected to said valve means, means defining a vacuum orifice through which said member is subjected to the pressure in said induction passage downstream of said throttle, means defining an air bleed orifice through which said member is subjected to atmospheric air, a bleed valve associated with said bleed orifice for controlling admission of air therethrough to vary'the control pressure created by induction passage vacuum received through said vacuum orifice and atmospheric air received through said air bleed orifice, pressure responsive means connected to said bleed valve and subjected to the pressure in said zone whereby upon an increase in pressure in said zone said pressure responsive means displaces said bleed valve to decrease admission of air through said bleed orifice and thereby decreases said control pressure to thereby cause said pressure responsive member to overcome the bias of said spring means and displace said valve means from said valve seat means and increase recirculation of exhaust gases, and second spring means biasing said bleed valve to permit increased admission of air through said bleed orifice whereby upon a decrease in pressure in said zone said second spring means displaces said bleed valve to increase admission of air through said bleed orifice and thereby increases said control pressure to thereby permit said first spring means to displace said valve means toward said valve seat means and decrease recirculation of exhaust gases.

3. An exhaust gas recirculation control valve assembly for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, and an exhaust gas recirculation passage having a first portion extending from said exhaust passage and a second portion extending to said induction passage downstream of said throttle, said control valve assembly comprising a valve body having an inlet for receiving exhaust gases from said first portion of said recirculation passage, an outlet for discharging exhaust gases to said second portion of said recirculation passage, valve seat means formed between said inlet and said outlet, and an orifice formed in said inlet, valve means associated with said valve seat means for controlling flow of exhaust gases therethrough, and control means for positioning said valve means to maintain a substantially constant pressure in the zone between said orifice and said valve seat means, said control means including spring means biasing said valve means toward engagement with said valve seat means, a hollow valve stem connected to said valve means and extending outwardly of said valve body, a pressure responsive assembly having a first diaphragm backing member secured to said valve stem, a diaphragm having a flexible inner portion defining an exhaust pressure chamber with said backing member and a flexible annular outer portion extending radially outwardly from said backing member, and a second diaphragm backing member defining an atmospheric air chamber with said inner portion of said diaphragm, a cover member defining a control pressure chamber with said outer portion of said diaphragm and said second backing member, said cover member having means for connecting said control pressure chamber to said induction passage downstream of said throttle, said diaphragm and said first backing member having openings for admitting atmospheric air to said atmospheric air chamber, said second backing member having an air bleed orifice for admitting atmospheric air'from said atmospheric air chamber to said control pressure chamber, a bleed valve connected to said inner portion of said diaphragm and associated with said air bleed orifice for controlling admission of air to said control pressure chamber, said hollow stem defining a passage connecting said exhaust pressure chamber to said zone, whereby upon an increase in pressure in said zone said inner portion of said diaphragm displaces said bleed valve to decrease admission of air to said control pressure chamber through said bleed orifice and thereby decreases the pressure in said control pressure chamber to thereby cause said pressure responsive member to overcome the bias of said spring means and displace said valve means from said valve seat means and increase recirculation of exhaust gases, and second spring means biasing said bleed valve to permit increased admission of air to said control pressure chamber through said bleed orifice whereby upon a decrease in pressure in said zone said second spring means displaces said bleed valve to increase admission of air to said control pressure chamber through said bleed orilice and thereby increases the pressure in said control pressure chamber to thereby permit said first spring means to displace said valve means toward said valve seat means and decrease recirculation of exhaust gases.

4. The method of operating an internal combustion" engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, an exhaust gas recirculation passage extending from said exhaust passage to said induction passage downstream of said throttle, a valve controlling exhaust gas flow through said exhaust gas recirculation passage, and pressure responsive means for positioning said valve, said method comprising the steps of sensing the pressure in said induction passage downstream of said throttle,

opening an air bleed to increase the pressure sensed from said induction passage and thereby-create a control pressure signal,

subjecting said pressure responsive means to said control pressure signal,

varying the opening of said air bleed in inverse rela tionship with the pressure in said exhaust gas recirculation passage upstream of said valve whereby said pressure responsive means operates said valve to create a zone of substantially constant pressure in said exhaust gas recirculation passage,

and flowing exhaust gases through an orifice into said zone,

whereby the rate of exhaust gas flow through said exhaust gas recirculation passage varies in accordance with the exhaust gas pressure in said exhaust passage and is thereby proportional to the rate of air flow through said induction passage.

5. An exhaust gas recirculation control valve assembly for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controllipg air flow therethrough, an exhaust passage for exhaust gas flow from the engine, and an exhaust gas recirculation passage having a first portion extending from said exhaust passage and a second portion extending to said induction passage downstream of said throttle, said control valve assembly comprising a valve body having an inlet for receiving exhaust gases from said first portion of said recirculation passage, an outlet for discharging exhaust gases to said second portion of said recirculation passage, valve seat means formed between said inlet and said outlet, and an orifice formed in said inlet, valve means associated with said valve seat means for controlling flow of exhaust gases therethrough, and control means for positioning said valve means to maintain a substantially constant pressure in the zone between said orifice and said valve seat means, said control means including spring means biasing said valve means toward engagement with said valve seat means, a pressure responsive member connected to said valve means, a housing defining a control pressure chamber having a vacuum orifice through which said control pressure chamber is subjected to the pressure in said induction passage downstream of said throttle and an air bleed orifice through which said control pressure chamber is subjected to atmospheric air, a bleed valve associated with said bleed orifice for controlling admission of air therethrough to vary the control pressure created in said control pressure chamber by induction passage vacuum received through said vacuum orifice and atmospheric air received through said air bleed orifice, means for subjecting said pressure responsive member to the control pressure in said control pressure chamber, a diaphragm connected to said bleed valve and subjected to the pressure in said zone whereby upon an increase in pressure in said zone said diaphragm displaces said bleed valve to decrease admission of air to said control pressure chamber through said bleed orifice and thereby decreases said control pressure to thereby cause said pressure responsive member to overcome the bias of said spring means and displace said valve means from said valve seat means and increase recirculation of exhaust gases, and second spring means biasing said bleed valve to permit increased admission of air to said control pressure chamber through said bleed orifice whereby upon a decrease in pressure in said zone said second spring means displaces said bleed valve to increase admission of air to said control pressure chamber through said bleed orifice and thereby increases said control pressure to thereby permit said first spring means to displace said valve means toward said valve seat means and decrease recirculation of exhaust gases 

1. An exhaust gas recirculation control valve assembly for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, and an exhaust gas recirculation passage having a first portion extending from said exhaust passage and a second portion extending to said induction passage downstream of said throttle, said control valve assembly comprising a valve body having an inlet for receiving exhaust gases from said first portion of said recirculation passage, an outlet for discharging exhaust gases to said second portion of said recirculation passage, valve seat means formed between said inlet and said outlet, and an orifice formed in said inlet, valve means associated with said valve seat means for controlling flow of exhaust gases therethrough, and control means for positioning said valve means to maintain a substantially constant pressure in the zone between said orifice and said valve seat means, said control means including spring means biasing said valve means toward engagement with said valve seat means, a pressure responsive member connected to said valve means, means defining a vacuum orifice through which said member is subjected to the pressure in said induction passage downstream of said throttle, means defining an air bleed orifice through which said member is subjected to atmospheric air, a bleed valve associated with one of said orifices for controlling flow therethrough and thus regulating the control pressure created by induction passage vacuum received through said vacuum orifice and atmospheric air received through said air bleed orifice, pressure responsive means connected to said bleed valve and subjected to the pressure in said zone whereby upon an increase in pressure in said zone said pressure responsive means displaces said bleed valve to decrease said control pressure to thereby cause said pressure responsive member to overcome the bias of said spring means and displace said valve means from said valve seat means and increase recirculation of exhaust gases, and second spring means biasing said bleed valve to increase said control pressure whereby upon a decrease in pressure in said zone said second spring means displaces said bleed valve to increase said control pressure to thereby permit said first spring means to displace said valve means toward said valve seat means and decrease recirculation of exhaust gases.
 2. An exhaust gas recirculation control valve assembly for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, and an exhaust gas recirculation passage having a first portion extending from said exhaust passage and a second portion extending to said induction passage downstream of said throttle, said control valve assembly comprising a valve body having an inlet for receiving exhaust gases from said first portion of said recirculation passage, an outlet for discharging exhaust gases to said second portion of said recirculation passage, valve seat means formed between said inlet and said outlet, and an orifice formed in said inlet, valve means associated with said valve seat means for controlling flow of exhaust gases therethrough, and control means for positioning said valve means to maintain a substantially constant pressure in the zone between said orifice and said valve seat means, said control means including spring means biasing said valve means toward engagement with said valve seat means, a pressure responsive member connected to said valve means, means defining a vacuum orifice through which said member is subjected to the pressure in said induction passage downstream of said throttle, means defining an air bleed orifice through which said member is subjected to atmospheric air, a bleed valve associated with said bleed orifice for controlling admission of air therethrough to vary the control pressure created by induction passage vacuum received through said vacuum orifice and atmospheric air received through said air bleed orifice, pressure responsive means connected to said bleed valve and subjected to the pressure in said zone whereby upon an increase in pressure in said zone said pressure responsive means displaces said bleed valve to decrease admission of air through said bleed orifice and thereby decreases said control pressure to thereby cause said pressure responsive member to overcome the bias of said spring means and displace said valve means from said valve seat means and increase recirculation of exhaust gases, and second spring means biasing said bleed valve to permit increased admission of air through said bleed orifice whereby upon a decrease in pressure in said zone said second spring means displaces said bleed valve to increase admission of air through said bleed orifice and thereby increases said control pressure to thereby permit said first spring means to displace said valve means toward said valve seat means and decrease recirculation of exhaust gases.
 3. An exhaust gas recirculation control valve assembly for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, and an exhaust gas recirculation passage having a first portion extending from said exhaust passage and a second portion extending to said induction passage downstream of said throttle, said control valve assembly comprising a valve body having an inlet for receiving exhaust gases from said first portion of said recirculation passage, an outlet for discharging exhaust gases to said second portion of said recirculation passage, valve seat means formed between said inlet and said outlet, and an orifice formed in said inlet, valve means associated with said valve seat means for controlling flow of exhaust gases therethrough, and control means for positioning said valve means to maintain a substantially constant pressure in the zone between said orifice and said valve seat means, said control means including spring means biasing said valve means toward engagement with said valve seat means, a hollow valve stem connected to said valve means and extending outwardly of said valve body, a pressure responsive assembly having a first diaphragm backing member secured to said valve stem, a diaphragm having a flexible inner portion defining an exhaust pressure chamber with said backing member and a flexible annular outer portion extending radially outwardly from said backing member, and a second diaphragm backing member defining an atmospheric air chamber with said inner portion of said diaphragm, a cover member defining a control pressure chamber with said outer portion of said diaphragm and said second backing member, said cover member having means for connecting said control pressure chamber to said induction passage downstream of said throttle, said diaphragm and said first backing member having openings for admitting atmospheric air to said atmospheric air chamber, said second backing member having an air bleed orifice for admitting atmospheric air from said atmospheric air chamber to said control pressure chamber, a bleed valve connected to said inner portion of said diaphragm and associated with said air bleed orifice for controlling admission of air to said control pressure chamber, said hollOw stem defining a passage connecting said exhaust pressure chamber to said zone, whereby upon an increase in pressure in said zone said inner portion of said diaphragm displaces said bleed valve to decrease admission of air to said control pressure chamber through said bleed orifice and thereby decreases the pressure in said control pressure chamber to thereby cause said pressure responsive member to overcome the bias of said spring means and displace said valve means from said valve seat means and increase recirculation of exhaust gases, and second spring means biasing said bleed valve to permit increased admission of air to said control pressure chamber through said bleed orifice whereby upon a decrease in pressure in said zone said second spring means displaces said bleed valve to increase admission of air to said control pressure chamber through said bleed orifice and thereby increases the pressure in said control pressure chamber to thereby permit said first spring means to displace said valve means toward said valve seat means and decrease recirculation of exhaust gases.
 4. The method of operating an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, an exhaust gas recirculation passage extending from said exhaust passage to said induction passage downstream of said throttle, a valve controlling exhaust gas flow through said exhaust gas recirculation passage, and pressure responsive means for positioning said valve, said method comprising the steps of sensing the pressure in said induction passage downstream of said throttle, opening an air bleed to increase the pressure sensed from said induction passage and thereby create a control pressure signal, subjecting said pressure responsive means to said control pressure signal, varying the opening of said air bleed in inverse relationship with the pressure in said exhaust gas recirculation passage upstream of said valve whereby said pressure responsive means operates said valve to create a zone of substantially constant pressure in said exhaust gas recirculation passage, and flowing exhaust gases through an orifice into said zone, whereby the rate of exhaust gas flow through said exhaust gas recirculation passage varies in accordance with the exhaust gas pressure in said exhaust passage and is thereby proportional to the rate of air flow through said induction passage.
 5. An exhaust gas recirculation control valve assembly for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, an exhaust passage for exhaust gas flow from the engine, and an exhaust gas recirculation passage having a first portion extending from said exhaust passage and a second portion extending to said induction passage downstream of said throttle, said control valve assembly comprising a valve body having an inlet for receiving exhaust gases from said first portion of said recirculation passage, an outlet for discharging exhaust gases to said second portion of said recirculation passage, valve seat means formed between said inlet and said outlet, and an orifice formed in said inlet, valve means associated with said valve seat means for controlling flow of exhaust gases therethrough, and control means for positioning said valve means to maintain a substantially constant pressure in the zone between said orifice and said valve seat means, said control means including spring means biasing said valve means toward engagement with said valve seat means, a pressure responsive member connected to said valve means, a housing defining a control pressure chamber having a vacuum orifice through which said control pressure chamber is subjected to the pressure in said induction passage downstream of said throttle and an air bleed orifice througH which said control pressure chamber is subjected to atmospheric air, a bleed valve associated with said bleed orifice for controlling admission of air therethrough to vary the control pressure created in said control pressure chamber by induction passage vacuum received through said vacuum orifice and atmospheric air received through said air bleed orifice, means for subjecting said pressure responsive member to the control pressure in said control pressure chamber, a diaphragm connected to said bleed valve and subjected to the pressure in said zone whereby upon an increase in pressure in said zone said diaphragm displaces said bleed valve to decrease admission of air to said control pressure chamber through said bleed orifice and thereby decreases said control pressure to thereby cause said pressure responsive member to overcome the bias of said spring means and displace said valve means from said valve seat means and increase recirculation of exhaust gases, and second spring means biasing said bleed valve to permit increased admission of air to said control pressure chamber through said bleed orifice whereby upon a decrease in pressure in said zone said second spring means displaces said bleed valve to increase admission of air to said control pressure chamber through said bleed orifice and thereby increases said control pressure to thereby permit said first spring means to displace said valve means toward said valve seat means and decrease recirculation of exhaust gases. 